The proposed experiments are directed toward a quantitative study of reflex responses arising upon application of chemoreceptor stimuli at the ileum. The ileum of cats under chloralose-urethane anesthesia, neurally intact but isolated with regard to circulation, will be perfused at constant flow with warmed cat blood. A control perfusate will alternate at ten minute intervals with a stimulus perfusate, the latter consisting of hypoxic or hypercapnic perfusion fluid, or a combination of these. The use of a perfusion apparatus with two gas exchange reservoirs will allow chemoreceptor stimuli to be applied as step functions, thus maximizing the responses. The partial pressures in the stimulus fluids will approximate values found in normal venous blood or arterial blood after 3 min of asphyxia, these being 40 and 25 mm Hg for oxygen and 40 and 70 mm Hg for carbon dioxide. Respiratory responses to be determined are tidal volume, breathing rate and minute volume. Cardiovascular responses to be determined are arterial pressure and heart rate. The effect of artificial ventilation on the reflex responses will also be studied. Deviations from control values will be calculated both the initial (peak) responses and from responses at 10 min (equilibrium). The data will lend themselves to calculation of significance via the t test for paired data.